Gene flow and genetic diversity in naturally fragmented metapopulations of deep-sea hydrothermal vent animals.

The ephemeral nature of deep-sea hydrothermal vents is expected to favor species with good colonization abilities, high dispersal rates, and rapid individual growth rates. Studies of gene flow in vent-endemic species provide glimpses into modes and patterns of dispersal. For some species, gene flow occurs without geographical bias (i.e., island model); their dispersal capabilities probably exceed the sampled geographical range. For other species, genetic differentiation increases with geographical distance (isolation-by-distance model) and suggests a stepping-stone mode of dispersal between neighboring vents. Genetic subdivision in a third group of species is associated with geographical offsets between contiguous segments of a ridge axis. These species all possess a free-living larval stage and average rates of gene flow (Nm) exceeding the critical value of one. In contrast, an amphipod that broods its young shows evidence for isolation-by-distance along a ridge axis and nearly complete isolation between distinct ridge axes. Early successional species (i.e., those that rapidly establish populations at nascent vents) also have high levels of genetic variability that probably results from a larger global population size. Bivalve species, which are restricted to a few of the known vent sites, appear at a later successional stage and have lower levels of variability. The relative successional position (early versus late) and overall abundance of a species may play significant roles in determining the retention of genetic diversity in populations inhabiting these ephemeral environments.